This invention relates to the recovery of silver from waste photographic solutions. Although typically most of the silver removed from silver-halide imaging elements occurs in the fixing step, leaching out during the fix step, some quantity of silver is also removed in the preceding steps, such as the developer and bleach steps. There are three principle means that are used for silver recovery: (1) chemical precipitating agents, (2) electrolysis and (3) the use of a galvanic cell or metalxe2x80x94metal exchange. This invention relates to a method employing metalxe2x80x94metal exchange.
Recovery of silver ions from solution can occur when the silver ions come into contact with a metal having a higher electropositive oxidation potential than silver. The reaction proceeds as follows:
Mexe2x86x92Men++ne
nAg++nexe2x86x92nAg
where n is the valence of the metal (Me) that has a higher electropositive potential than the Ag+ ion.
Metals that fall into this category include Al, Fe, Zn. In most commercial recovery schemes, iron is the metal of choice, for health, safety, and environmental reasons and also for economic reasons. Typically, steel wool or chopped up steel wool is used in a canister. The silver-bearing waste solutions are contacted either in a batch mode or a continuous mode.
Several patents disclose methods and apparatus for recovering silver by the metalxe2x80x94metal exchange reaction. For example, U.S. Pat. Nos. 2,194,056; 3,705,716 and 4,065,313 disclose methods and apparatus for recovering silver from waste photographic processing solutions by an exchange reaction with a metal, such as iron or copper, that is more electropositive than silver. European publication EP27769 discloses a method of monitoring the redox potential of the silver-bearing solution while the metalxe2x80x94metal exchange reaction is taking place. This is done to minimize the re-dissolution of silver when the ferrous ions are oxidized to Fe3+ ions.
U.S. Pat. Nos. 5,188,662, and 5,310,629 disclose substrates coated with a hydrophilic colloid containing physical development nuclei to remove soluble silver from developer solutions during the development step, in order to maintain the performance of the developer solution. Useful hydrophilic colloids include proteins, gelatin, and polysaccharides such as dextrin, to name a few. In U.S. Pat. No. 5,188,662, the development nuclei are noble metals that are less electropositive. In U.S. Pat. No. 5,310,629, the development nuclei also include sulfides of metals. While this technology is also capable of use to recover silver from waste photoprocessing solutions, noble metals are substantially more expensive than the electropositive metals listed above. Sulfides of metals are, in general, undesirable because of the possibility of developing an odor due to generation of hydrogen sulfide. Also, since the object of the aforementioned patents is to remove silver from functioning developing solutions, one would not want to use electropositive metals, which could ionize in the developer solution and change its functionality. Japanese patent application 01-050047 discloses a cleaning film that comprises a substrate coated with a hydrophilic colloid (like gelatin) containing a hydrophobic organic compound that is capable of adsorbing silver on the substrate. This film is also used to remove soluble silver from functioning developer solutions.
In view of the above, the prior art that discloses the removal of silver ion from solution in the presence of a hydrophilic colloid either use a crystallization phenomenon or simple physical adsorption, which methods are less effective and more expensive than the metalxe2x80x94metal exchange reaction.
In a typical photoprocessing operation, silver-bearing waste from the operation is passed through a cartridge containing an electropositive metal. For example, steel wool in a canister is known for this purpose.
It is highly preferred that the hydrophilic binder used in the present invention be crosslinked to some degree, to prevent disintegration/dissolution of the binder when it comes into contact with an aqueous solution. For example when polyvinyl alcohol is used as the hydrophilic colloid, it can be crosslinked with sodium borate (Borax). In the case of the preferred hydrophilic colloid, gelatin, several types of crosslinkers or hardeners are known in the art. These are described in xe2x80x9cThe Theory of the Photographic Processxe2x80x9d 4th Ed., Ed. T. H. James, pg. 77-87, 1977. The class of inorganic crosslinkers includes salts of chromium and some salts of aluminum. These typically crosslink via the free carboxylic acids in gelatin, and the degree of crosslinking is pH sensitive and also reversible. It may not be desirable, however, to use these salts for absorbents because of the impact these materials have on the environment. Organic crosslinkers are, therefore, preferred. The organic crosslinkers act via the xcex5-amino function of lysine and hydroxylysine. There are, on the average, about 0.35-0.4 mmol of lysine per gram of dry gelatin and about 20% of that amount of hydroxylysine per gram of dry gelatin. Classes of organic crosslinkers include, but are not limited to, aldehydes and blocked aldehydes, ketones, carboxylic and carbamic acid derivatives, sulfonate esters and sulfonyl halides, s-triazines, epoxides, aziridines, isocyanates, carbodiimides and isoxazolium salts. Polymeric crosslinkers are generic polymer molecules bearing one or more of the above moieties in their chain. The selection of the crosslinker type depends on the efficacy of the crosslinking, its toxicity in the native state, the residuals in the absorbent, and cost.
It has been found that the problems encountered using fine metal particles for the recovery of silver from a silver-bearing photographic waste solution can be alleviated by encapsulating or embedding metal particles in a matrix of a hydrophilic colloid that allows for the permeability of water and silver ions. When contacted with the silver-bearing photographic waste solution, the hydrophilic colloid will imbibe water and swell. At the same time, the silver ions can permeate the colloidal matrix and come into contact with the metal particles, preferably iron particles, at which point a metalxe2x80x94metal exchange reaction takes place to reduce the silver ion to silver metal.
By embedding, dispersing, or encapsulating the metal particles within a hydrophilic colloid, the reduced silver is precipitated within a colloidal matrix, whereby the hydrophilic colloid also functions as a filter. The hydrophilic colloid/metal combination can take various physical forms or shapes, such as, for example, a coating or coated film, a coated substrate, granules, a block of variable shape, or a flow-through filter. The recovery element may be attached/affixed to a surface or other object or remain unattached. If used in a filter, the filter may be of any conventional design. See, for example, U.S. Pat. No. 5,310,629, hereby incorporated by reference.
The advantages of the present invention over other methods of recovering silver include the fact that, since it involves metalxe2x80x94metal exchange, it allows the use of a metal with a high specific surface area, but without the problems associated with handling of powders as well as the oxidation of such powders over time. It also obviates the need for a filter to separate the silver from the solution. Finally, it increases the ease of handling the recovered silver, i.e., one can deal with granules or coatings (containing relatively little or no liquid) rather than slurries containing finely divided silver and oxidation products of the metals themselves. Another advantage is that, by encapsulating the fine metal particles within the hydrophilic colloid, it was surprisingly found that the amount of metal needed to maintain the same reaction kinetics is much less than that needed when the metal is in the form of chopped or shredded wool, as currently used in the trade.
As indicated above, the present invention involves the recovery of silver from a silver-bearing photographic waste solution by encapsulating or embedding fine metal particles in a matrix made from a hydrophilic colloid, which hydrophilic colloid that allows the permeability of water and silver ions. When the silver ions in the waste solution permeates the colloidal matrix and comes into contact with the metal particles, a metalxe2x80x94metal exchange reaction takes place in which the silver ions are reduced to silver metal.
The following criteria normally apply to the hydrophilic colloid. First, the metal particles should be easily dispersed in a solution of the colloid and should remain dispersed as the colloid is dried. That is, the colloid should not cause the particles to flocculate or clump. Second, the hydrophilic colloid should not dissolve in the aqueous waste solution it is used to treat, as a function of time of use. Otherwise the metal particles or the exchanged silver will be released into solution. Third, the hydrophilic colloid must swell with the solvent (water) and be permeable to the silver and metal ions. The metal is oxidized to metal ions, which will diffuse out of the binder matrix.
By dispersing metal particles within a hydrophilic colloid, the reduced silver is precipitated within the hydrophilic colloid matrix, allowing the hydrophilic colloid to effectively function as, or effectively replace and obviate the need for, a filter. The hydrophilic colloid/metal combination can take several physical forms, such as a coated film, or granules, coated substrate, flow-through filter, and the like. Various conventional forms for a silver recovery element are disclosed in the prior art, mentioned above. In a preferred embodiment, chopped or shredded steel wool is packed into a hollow canister at the exit of which is a filter. Silver containing waste solutions are sent through the canister. Reduced silver particles are trapped by the filter. In order to reduce the silver concentration in the waste to desired levels, the solutions may be passed through more than one canister, sequentially.
The metals used to recover silver must have a higher electropositive oxidation potential than silver. Examples of such metals are tin, copper, lead, nickel, cadmium, zinc, aluminum, and iron. For cost and health reasons, the more preferable metals are aluminum, zinc and iron. Aluminum, however, is less than ideal because it may become passivated due to the formation of a surface layer of aluminum oxide. Iron is the most preferable among all the metals.
The particle size of the metal can vary. If the size is too large, however, it is difficult to disperse or encapsulate and, because the specific surface area is small, the reaction kinetics may be slower. Thus, it is preferred that the particles size is smaller than 1 mm. On the other hand, if the size is too small, the particles may tend to be more reactive than optimal and, as a result, may become passivated or present an explosion hazard during preparation. It is preferred, therefore, that the particles are greater than 5 xcexcm in average diameter. The more preferred average particle size (diameter) is between 30 xcexcm and 500 xcexcm. The most preferred particle size is between 30 xcexcm and 100 xcexcm.
The hydrophilic colloids that can be used in this invention can also be referred to as binders, since they must not disintegrate or dissolve upon contact with a water-based solution, including the waste solution to be treated. Hydrophilic colloid binders include, but are not limited to, water soluble polymers such as gelatin, gelatin derivatives, maleic acid anhydride copolymers, cellulose compounds (such as carboxymethyl cellulose, hydroxyethyl cellulose, cellulose acetate butyrate, diacetyl cellulose or triacetyl cellulose), synthetic hydrophilic polymers (such as polyvinyl alcohol, poly-N-vinylpyrrolidone, acrylic acid copolymers, polyacrylamides, partially hydrolyzed acetate and polyacrylate acid esters), and derivatives, copolymers, mixtures or blends of all of the above polymers, and the like.
Gelatin is the most preferred binder. For example, gelatin for use in the present invention may be obtained by alkali treatment of collagen, by immersing the latter in an alkali bath. It may be obtained from acid treatment of collagen, where the collagen is immersed in an acid bath prior to extraction. It may be obtained by collagen treated in both baths and, finally, it may be obtained by enzyme treatment. The main differences in these gelatins is in their isoelectric point (i.e.p.), which is the pH at which the gelatin molecule has no net charge, due to the balance of the positive and negative charges. Acid processed gelatin has an i.e.p. from 7-9.0, whereas alkali processed gelatin has an i.e.p. at about 5.0. The sources of collagen used to make the gelatin can vary. Typical sources are bovine ossein, hide, pig skin and poultry ossein. Depending on the amount of hydrolysis, the mean molecular weight of the gelatin can vary. Gelatin derivatives can also be used. These include gelatins derivatized with aromatic sulfonyl chlorides, carboxylic acid chlorides, carboxylic acid anhydrides, aryl isocyanates, 1,4-diketones, etc. Depending on the groups that are added to the gelatin, the i.e.p. of the resultant gelatin, can be above or below that of the parent gelatin material.
It is highly preferred that the hydrophilic binder used in the present invention be crosslinked to some degree, to prevent disintegration/dissolution of the binder when it comes into contact with an aqueous solution. For example when polyvinyl alcohol is used as the hydrophilic colloid, it can be crosslinked with sodium borate (Borax). In the case of the preferred hydrophilic colloid, gelatin, several types of crosslinkers or hardeners are known in the art. These are described in xe2x80x9cThe Theory of the Photographic Processxe2x80x9d 4th Ed., Ed. T. H. James, pg. 77-87, 1977, hereby incorporated by reference. The class of inorganic crosslinkers includes salts of chromium and some salts of aluminum. These typically crosslink via the free carboxylic acids in gelatin, and the degree of crosslinking is pH sensitive and also reversible. It may not be desirable, however, to use these salts for absorbents because of the impact these materials have on the environment. Organic crosslinkers are, therefore, preferred. The organic crosslinkers act via the xcex5-amino function of lysine and hydroxylysine. There are, on the average, about 0.35-0.4 mmol of lysine per gram of dry gelatin and about 20% of that amount of hydroxylysine per gram of dry gelatin. Classes of organic crosslinkers include, but are not limited to, aldehydes and blocked aldehydes, ketones, carboxylic and carbamic acid derivatives, sulfonate esters and sulfonyl halides, s-triazines, epoxides, aziridines, isocyanates, carbodiimides and isoxazolium salts. Polymeric crosslinkers are generic polymer molecules bearing one or more of the above moieties in their chain. The selection of the crosslinker type depends on the efficacy of the crosslinking, its toxicity in the native state, the residuals in the absorbent, and cost.
The amount of crosslinker used may depend on the degree of stability (against dissolution of the binder) that is desired. For purposes of this invention, we define the xe2x80x9ceffective molexe2x80x9d of crosslinker as the moles (number of molecules divided by the Avogadro Number) of all species that can react with two xcex5-amine sites in gelatin. Thus, for a simple crosslinker like formaldehyde, the effective moles is equal to the actual moles, whereas for a polymeric crosslinker the effective moles is calculated based on the total moles of the monomers that act as crosslinkers. Typically, it is found that in order for the gelatin to be stable in temperatures up to 50xc2x0 C., it is preferably that the amount of crosslinker should be greater than 10 xcexcmole/gm gelatin. However, in addition to preventing dissolution of gelatin, the crosslinker can also minimize the swell of the gelatin with the silver bearing aqueous solution. The swell of the gelatin is defined as the increase in weight of the gelatin granule, due to imbibation of the solvent (water) into the gelatin matrix. The higher the amount of crosslinker, usually the less is the ability to swell. In many silver recovery applications, it may be undesirable to recover the silver along with a lot of water, because of the energy subsequently required to recover the silver. Thus, in order to keep the swell to a minimum or optimal amount, the amount of crosslinker can be as high as, for example, 350 xcexcmole/gm of gelatin. At crosslinker levels higher than that, the decrease in swell with increasing crosslinker is small and may not be worth the cost of the extra crosslinker. The preferred range of crosslinker for gelatin bearing systems is from 50 to 250 xcexcmole/gm of gelatin.
The amount of metal with respect to the amount of hydrophilic colloid can be optimized for the cost of the material and the performance. If the volume ratio of iron to hydrophilic colloid is too low, then the amount of hydrophilic colloid that is needed becomes prohibitively expensive. Furthermore, due to the swelling of the hydrophilic colloid, the amount of liquid imbibed per gram of silver recovered becomes high. If the metal to hydrophilic colloid volume ratio is too high, then some of the metal may be exposed at the surface of the hydrophilic colloid. In that case the silver will be precipitated outside the hydrophilic colloid matrix and the efficacy of the silver filter is reduced. It is found that preferably the volume of the total body of functional material (i.e., combined metal and hydrophilic colloid matrix) occupied by the metal is in the range of from 5% to 35%. The most preferred range is from 10% to 30%.
The silver-recovery element comprising the hydrophilic-colloid matrix/metal functional material can be in several forms. For example. it can be in the form of granules of hydrophilic colloid, having embedded in it metal particles. The functional material can also be coated onto an inert substrate or a substrate having a different function, wherein the coated substrate is contacted with the photoprocessing solution.
In one embodiment, substantially all of the metal particles furthest from the surface of the matrix are not further than 10 mm. Preferably, not more than 1% (by weight), more preferably none, of the particles should be further than 10 mm. If this distance is greater than 5 mm, then the kinetics of diffusion, with respect to the silver and metal ions, tends to slow down, and the use of the metal is not as efficient. If in the form of granules, we have found that, in order for the metal to be used most efficiently, preferably more than 90% of the granules should be able to pass through a screen with a 10 mm opening.
As indicated above, the hydrophilic colloid/metal material can also be coated on a substrate. Any type of substrate can be used, provided the substrate has adequate dimensional stability when contacted with the processing solution. Plastic supports are preferred. Examples of these are cellulose derivatives including cellulose diacetate, cellulose triacetate and the like; polyamides; polycarbonates; polyesters, particularly polyethylene terephthalate and polyethylene naphthalate; polystyrene, polypropylene, polyethylene, polysulfone, polyarylates, polyether imides, and the like. Paper based supports can also be used, particularly resin-coated papers used in photographic and printing industry. Supports for photographic elements are described in Research Disclosure, December 1989, Item 308,119 published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire P010 7DQ England, section XVII.
To make the silver recovery element in which the functional material is coated on a substrate, one way is to apply a solution of the hydrophilic colloid and iron to a support by a standard coating technique such as slide coating, bar coating, roll coating, knife coating, curtain coating, rotogravure coating, spraying, and dipping. In order to increase the efficiency of recovery, a high level of hydrophilic colloid and metal per unit area should be laid down. In order to alleviate the problems associated with curl for such high laydowns of hydrophilic colloid, it is preferred that both sides of the support be coated with hydrophilic colloid and iron. The amount of hydrophilic colloid desired is from 1000 mg/ft2 to 10,000 mg/ft2 on each side of the support. In accordance with the preferred amount of metal in the material, the amount of metal to be laid down is from 5 to 35% of the volume of the material (hydrophilic colloid plus metal) laid down. In addition, the hydrophilic-colloid/metal coating may optionally have an overcoat with gelatin or another hydrophilic binder, with no metal being present in the overcoat. This may be done, for example, to ensure that the recovered silver is embedded within the coating and not on the surface of the coating.
When carrying out the process of the invention, the step of contacting the waste solution with the silver recovery element should be for a time sufficient to reduce the concentration of silver in the waste solution to a desired level. The desired final silver concentration and treatment time are readily determinable, and can be influenced by factors such as solution flow rate, contact time, starting silver concentration, and the design and efficiency of the recovery element, as can be preselected by the skilled artisan. In one embodiment, the treatment time to reduce silver concentration from about 5000 mg/liter to about 5 mg/liter is about 10 hours. Suitably, in one embodiment, the process can be carried out at a temperature in the range of from about 20xc2x0 C. to about 50xc2x0 C. The process can be conducted at any pressure in the range of from about atmospheric pressure for a stated set of reaction conditions to about 100 atmospheres. In one embodiment, a recovery element can comprise a canister containing the granules described above that is positioned before the drain for the waste solution which allows waste solution to flow from the photoprocessor, contact the granules for a sufficient time, so as to reduce the silver ion concentration, and subsequently exit the canister substantially silver free. When the granules have been consumed of the metal, they can be sent to a refiner to recover the precious metal. In another embodiment, the recovery element can be snuggly positioned in a silver recovery canister comprising a housing and having ports for the respective intake of waste solution and the discharge of treated waste solution. Optionally, a recirculation system containing a silver recovery element of the invention, developer solution can allow partial bypass of the silver recovery element. The canisters can contain either of the two preferred embodiments of the silver recovery materials of this invention.
Alternately, waste solution can be taken from miniprocessor or a kiosk type photoprocessor, where there is no access to drains. The solution is collected in a container, till the photoprocessor is serviced and the waste solution is hauled away or dumped. The invention in the form of granules can be contained in a bag of a coarse knit fabric, to allow easy transport of solution to the granules. Alternately, the a coating described above can be placed in the container. The silver ions in the solution is continuously reduced till the time of servicing. At the service time the granules or the coating can be removed, optionally dried, and sent to the photofinisher for recovery and provided to a tank containing the recovery element by means of a pump. An optional flow control valve may be employed.
In a preferred embodiment, the silver recovery element according to the present invention is used in a stand-alone unmanned photochemical processor. Such stand-alone photochemical processors are disclosed in commonly-assigned copending application U.S. Ser. No. 09/822,963, hereby incorporated by reference in its entirety.
The present invention is further illustrated by the following examples of its practice and design.